Dimethyl sulfoxide (DMSO) added to agar overlays during plaque assays of simian virus 40 (SV40) in CV1 monkey cells increases the plaque size and number and enables plaques to be read several days earlier than usual. DMSO appears to act during development of plaques, perhaps by causing cell lysis at smaller burst sizes in the presence of near-lethal DMSO concentrations. It does not act synergistically in determining virus inactivation with UV light and is equally effective on wild type and a late mutant of SV40.